Electrical drive systems using electric motors to power the end-use function are in wide use for propulsion, machine drives, conveyor lines, chemical processing, material handling applications and the like. A very small sampling of exemplary drive systems appears in U.S. Pat. Nos. 3,845,366 (Metzler et al.) and 4,166,238 (Binner).
Certain types of material handling machines incorporate electric motor drive systems for moving the machine from location to location, for moving a machine "substructure" on the machine itself and for moving loads of the type the machine is designed to handle. An example of such a material handling machine is an overhead travelling crane (OTC) used in factories, steel handling bays and the like for lifting and placing loads. Such a crane traverses along a pair of elevated main rails which are parallel and spaced apart, usually by several yards. A pair of crane bridge girders extends between the rails and there are driven wheels mounted at either end of the girders for supporting the crane atop the rails. And the girders themselves have rails on them.
A substructure called a "trolley" is mounted on the girder rails and traverses the width of the bridge under motive power. A load hoist is mounted on the trolley and includes a powered hoist/lower "rope drum" or drums about which steel cable is spirally wrapped. The cable is connected to a load-lifting hook, sling, bucket, magnet or the like. With the foregoing arrangement, the operator (who usually rides in a cab which is attached to and moves with the bridge) can pick up, move and deposit a load anywhere in the area travelled by the crane. Other, somewhat less common operating options include radio-controlled cranes operable from the ground or other remote location and operator cabs which are trolley, rather than bridge, mounted.
An exemplary overhead crane employs two electric-motor traverse drive systems, one each for the bridge and trolley traverse drives. A third electric-motor drive system is used for hoisting and lowering loads. Such drive systems may be powered by direct current (DC) or alternating current (AC). While DC drive systems were almost universally used in older steel mills and the like, AC variable frequency drive systems are becoming increasingly common, at least in part because of the advantages of precision control and design flexibility which they offer. In a variable frequency drive system, motor speed is a function of the frequency of the electrical voltage applied to it. Examples of AC variable frequency drive systems (used for hoist drives) are described in U.S. Pat. Nos. 4,965,847 (Jurkowski et al.) and 5,077,508 (Wycoff et al.). The leading manufacturer of overhead cranes and AC drive systems therefor is Harnischfeger Industries, Inc. of Milwaukee, Wisconsin. One such AC drive system is sold under the trademark SMARTORQUE.RTM. and the invention involves a modification of a known type of SMARTORQUE.RTM. controller.
Hoist, bridge and trolley drives are operated by an electrical controller coupled to an operator-manipulated master switch in the cab. Such master switch has a handle with a neutral position and a series of positions in each of two directions from neutral. The handle thus controls drive speed in either of two directions. And, subject to the limitation described below, the farther the handle is moved away from the neutral position, the faster the drive moves the load, e.g., the bridge or trolley or the load suspended from the hoist. And the counterpart is that as a master switch is moved toward its neutral position, the drive moves the load more slowly.
In either event, the electric motor and controller "ramp" the speed change so that such change occurs no more rapidly than the maximum predetermined rate set by the slope of the ramp. The quoted expression derives its name from the fact that when depicted on a two-axis graph, the lines representing rates of acceleration and deceleration and are therefore ramp-like in shape. Usually, the control manufacturer sets such rates--they are not changed in day-to-day crane operations.
Before setting forth additional background information, an understanding of some fundamentals will be helpful. One such fundamental relates to alternating current (AC) motors and to some of the operating characteristics of a particular type of such AC motor, i.e., a squirrel cage motor. Another involves some operating principles of a type of motor controller known as an adjustable frequency inverter and the way such a controller is used with an AC squirrel cage motor. Yet another involves what is known as an asymmetrical load, i.e., a load which resists motor rotation in one direction and aids such rotation in the other, and how such a load affects the motor and the control scheme when a squirrel cage motor and adjustable frequency inverter are used to power the hoist drive.
A squirrel cage motor is so named because its rotor (formed with parallel conductors shorted together at their ends) resembles a squirrel cage in shape. In three phase configuration (the type used on crane drives), the motor has only three stator terminals. In other words, there are no rotor terminals "brought out" as with a wound rotor motor.
Such motors exhibit a characteristic "knee-shaped" speed/torque curve and in order to develop any output torque at all, the rotor (or armature, at it is often called) must rotate at a speed somewhat less than the rotational speed of the magnetic field in the stator. The latter speed is often referred to as the synchronous speed. In, for example, a squirrel cage motor having (at 60 Hz line frequency) a synchronous speed of 1,800 RPM, the running speed at rated output torque may be about 1,760 RPM. The difference between the synchronous speed and the running speed is referred to as the "slip" and often slip is expressed as a percent. References herein to "motor speed," "motor rotation" and the like refer to the rotor component of the motor.
The amount of slip is a function of load. Using the foregoing example, a completely unloaded motor may exhibit a running speed of, say, 1,780 RPM since a slight amount of slip is required to develop torque sufficient to turn the rotor. And for each motor, there is some maximum slip (or minimum rotor speed) at which the motor is incapable of exerting further torque. If such motor is loaded above that point, the motor "pulls out," i.e., stalls, and rotation stops.
Another fundamental relating to squirrel cage motors is that the rotational speed is, in general, a function of the frequency of the applied voltage. For example, a motor having a running speed at rated output torque and 60 Hz applied voltage of about 1,760 RPM would have a running speed of about 880 RPM at 30 Hz applied voltage. In recognition of this characteristic of a squirrel cage motor, the above-noted SMARTORQUE.RTM. AC drive system and other systems like it are called "inverters" and are configured to provide an output frequency (and voltage) which can be varied by changing the position of the master switch handle.
An electric motor drive system such as a crane hoist drive represents a somewhat unusual application. Unlike the bridge and trolley drives (and unlike many other types of drives not involving overhead travelling cranes), loads handled by the hoist drive are said to be asymmetrical. That is, the weight of the load either aids or resists motor rotation, depending upon the direction of load movement. More specifically, when the load is being hoisted, the force of gravity resists such upward movement and thus resists motor rotation. On the other hand, when the load is being lowered, the force of gravity (acting, of course, in a downward direction) aids motor rotation and acts in a way to urge the motor to run faster. This is sometimes referred to as an "overhauling load."
A crane hoist drive is not the only type of drive called upon to handle asymmetrical loads. Any drive moving a load between two elevations, e.g., up and down a ramp and by a reversing, sloping conveyor represents such a drive.
U.S. Pat. Nos. 4,965,847 (Jurkowski et al.) and 5,077,508 (Wycoff et al.) depict examples of electric motor drive systems for use on overhead travelling cranes and, more specifically, for use on the hoist systems of such cranes. Such systems power the motor by maintaining a substantially constant ratio between the motor applied voltage and the frequency of such voltage. As a result, the motor has a substantially constant stator current and, consequently, exhibits substantially constant torque over its entire speed range.
Such systems use scalar inverters and often incorporate means for motor speed "checking." If the motor speed is not within a predetermined range of error (of actual speed vs. commanded speed) and if such speed does not come within such range within a predetermined time, the system is shut down. (In contrast, a true closed loop system would be arranged to take corrective action and bring the motor speed within the predetermined range rather than shutting down the system.) The Jurkowski et al. patent is of particular interest since the invention is a substantial improvement upon the technology which it represents.
A disadvantage of systems of the type depicted in the Jurkowski et al. patent is that the time required to "check" whether the actual motor speed is within the predetermined range of permitted error is inordinately long, sometimes approaching one-half second. Yet another disadvantage of such systems is that the bias module producing a voltage representing actual speed uses a single voltage-to-motor speed (or volts per RPM) ratio in both the raising and lower directions even through the effect of slip is different for each direction. As a result, the predetermined range of permitted error is itself relatively broad and the maximum permitted error (between actual speed and commanded speed) is greater than desired for accurate control. The invention overcomes these disadvantages in a unique way.